fix: commenting with just slash-commands shows error

Description

This MR fixes a bug where submitting a comment that only contains slash-commands fails because of gitlab#346557 (closed). The fix is done by ignoring error messages that start with Commands only because that is a soft error that should only signal that the API call only caused the commands to be executed but not write a literal comment. That’s fine for us though as we only expect the former.

Related Issues

Workaround for gitlab#346557 (closed)

Resolves #357 (closed)

How has this been tested?

• Manually tested by myself
• Added an integration test in test/integration/issuable_webview.test.js

Screenshots (if appropriate)

Before	After

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap